An optical recording medium may be roughly classified into a read-only medium, such as a so-called compact disc, and a medium on which signals may be recorded, such as a magneto-optical disc. With these recording media, it is desired to raise the recording density further because digital video signals as recording signals are in need of a data volume several to ten times or more the data volume for digital audio signals, and also because there is a demand for further reducing the size of a product such as a player by reducing the size of the recording medium such as a disc even when recording the digital audio signals.
Meanwhile, the recording density of the optical recording medium is governed by the line density along the scanning direction of the recording tracks and the track density conforming to an interval between adjacent tracks in a direction perpendicular to the scanning direction. The opto-physical limits of the line and track densities are governed by the wavelength .lambda. of a light source and the numerical aperture NA of an objective lens. In general, a readout limit of 2NA/.lambda. is generally accepted as a spatial frequency during signal reproduction. For this reason, for realizing high density in an optical recording medium, it is thought to be necessary to shorten the wavelength .lambda. of the light source of the reproducing optical system, such as a semiconductor laser, and to increase the numerical aperture NA of the objective lens.
However, since there is also a limit in improving the wavelength .lambda. of the light source or the numerical aperture NA of the objective lens, research is are under way for improving the structure of the recording medium and the reading method to increase the recording density.
The density along the scanning direction of the laser beam (direction of the recording track) and the distance between adjacent pits (the so-called line recording density and the pit recording interval as a reciprocal thereof) are considered. Theoretically, the minimum pit interval q along the beam scanning direction is given by EQU q=.lambda./2NA
so that, for NA=0.5 and .lambda.=780 nm (0.78 .mu.m), the minimum pit interval q is 0.78 .mu.m, such that signals having the pit interval shorter than this value cannot be reproduced. This minimum pit interval is 1400 pits/ram in terms .of the recording line density. However, as discussed hereinabove, there is a demand for further increasing the line density.
In view of the above described status of the art, it is an object of the present invention to provide an optical recording medium in which the recording line density may be further improved to increase the capacity of the recording medium.